Aqueous polymer dispersion, preparation and use thereof

ABSTRACT

The present invention relates to an aqueous polymer dispersion comprising a copolymer of anhydride monomer units and vinyl monomer units, which copolymer has been subjected to an imidization reaction. The present invention further relates to a method for the production and use of such a dispersion.

The present invention relates to an aqueous polymer dispersioncomprising a copolymer of anhydride monomer units and vinyl monomerunits, which copolymer has been subjected to an imidization reaction.The present invention further relates to a method for the production anduse of such dispersion.

From EP-A-1060197 a method is known for the production of an aqueousdispersion of a styrene maleic anhydride copolymer (abbreviatedgenerally as SMA) which is partly imidized resulting in apoly(styrene-co-maleimide) [SMI] dispersion, to be used as a papersizing composition. The dispersion is applied in a top coat to a papersurface, with the aim of reducing the water-absorption properties of thepaper surface and of providing good inkjet printing properties.According to EP-A-1060197, the copolymer is synthesised using generallyknown processes. The copolymer is subjected to an imidization reaction,by contacting it with an aqueous solution of NH₃ or an amine (RNH₂), ata temperature of at least 95° C. and a pressure which is chosen such asto avoid boiling of the reaction mixture. The molar ratio of maleicanhydride monomer units and NH₃ or amine is selected between 1:0.8 and1:5. The imidization reaction is continued until a degree of imidizationof the maleic anhydride monomer units is obtained of at most 75%.Thereafter, the dispersion is applied in a top coat to the paper and thepaper is dried and calendared. It has, however, been found that whenprinting a paper with the aqueous copolymer dispersion in the top coatdisclosed in EP-A-1060197, the quality of the printing is insufficient.In particular, characters are printed insufficiently sharp; thedelimitation of different colours is insufficient as a consequence ofwhich colours fade into each other.

EP-A-1060197 also includes a comparative example of a polymer dispersionprepared according to DE-A-1720746. DE-A-1720746 describes an aqueousdispersion of a polymer obtained by subjecting an SMA containing aboutequal molar amounts of maleic anhydride and styrene to an imidizationreaction in an aqueous solution of NH₃ at a temperature above 120° C.The degree of imidization in the examples is not revealed. In thecomparative example of EP-A-1060197 made according to DE-A-1720746, theSMA is 89% imidized. It has been observed that this dispersion has avery wide particle size distribution and forms an unstable dispersion,forming a sediment already after standing for a short period.

There is thus a need for a new polymer dispersion composition with whichan improved printing quality may be achieved.

There is further a need for a process of producing such an polymerdispersion composition.

Therefore, the present invention aims at providing an aqueous polymerdispersion composition for use in a top coat for paper, with which animproved printing of the paper may be achieved.

This aim is achieved with the present invention, with the technicalfeatures of the characterising part of the independent claims.

The aqueous polymer dispersion of this invention comprises a copolymerof anhydride monomer units and vinyl monomer units, of which copolymerat least 90% of the moles of the anhydride monomer units are imidized.This polymer dispersion contains SMI in the form of discrete particleswhich can be called organic pigment.

Suitable anhydride monomers for use in the copolymer are, for example,α-β-unsaturated dicarboxylic anhydrides such as maleic anhydride,fumaric anhydride, citraconic anhydride, itaconic anhydride and mixturesthereof. Preferably the copolymer contains maleic anhydride monomerunits.

Suitable vinyl monomers for use in the copolymer include vinyl aromaticmonomers (such as styrene, α-methyl styrene, vinyl toluene and indene),mono-olefinic unsaturated hydrocarbons (such as ethylene, propylene andisobutylene), α-β-unsaturated carboxylic esters (such as acrylate esters(like ethylacrylate, butylacrylate and 2-ethylhexylacrylate),methacrylate esters (like methylmethacrylate, ethylmethacrylate and2-hydroxyethylmethacrylate) and maleate diesters (like dioctylmaleate)),halogenated olefins (such as vinyl chloride and vinylidene chloride) andmixtures thereof. Preferably the copolymer contains readily commerciallyavailable styrene or α-methyl styrene, although the presence of styrenemonomer units is most preferred.

It has been surprisingly found that when using a polymer dispersion inwhich at least 90 mole % of the anhydride monomer units have beenimidized, a coating composition is obtained which, when applied in acoating to a surface, gives an improved coating. It has been found thatif the imidization degree of the copolymer is at least 90%, theparticles have a glass transition temperature (Tg) of at least 160° C.and an improved mechanical strength. As a result of the improvedmechanical strength the particles are capable of withstandingdeformation forces occurring in the course of the calendaring processused after applying the coating to the paper surface. When applying thepolymer dispersion of this invention to a paper surface, a coating isobtained which consists of a plurality of small, discrete pigmentparticles showing good adhesion to each other and to the surface to becoated. It has further been found that upon printing a paper surfacethat has been coated with the aqueous polymer dispersion of thisinvention, an improved printing quality may be achieved: the charactersbeing well delimited from each other, superimposed successively appliedprinting layers being well delimited from each other, fading of adjacentcharacters and colours into each other being limited.

The observation of the improved printing quality is attributed to thefact that the coating is built up of a plurality of discrete particles,drainage channels being formed between the particles, the presence ofthese drainage channels taking care of fast removal of ink solvent. Whenanalysing the problems occurring with the state of the art coatings, ithas been observed that application of the coating has the effect that inthe course of the calendaring the partially imidized SMI copolymerparticles disintegrate, flow together and form a film. Film formationhas the effect that upon printing, the removal of solvent or dispersingagent for the ink is retarded and the printing quality is adverselyaffected. The organic pigment dispersion coating of the presentinvention to the contrary shows no tendency to film formation whenapplied in a coating, or during calendaring.

Preferably use is made of a copolymer in which the anhydride monomercontent ranges between 5-50 mole %, more preferably between 5 and 43mole %, even more preferably between 5-36 mole % and most preferably5-29 mole %, because of the end product properties.

The anhydride monomer content of 15-29 mole % is particularly preferred,as in this range the copolymer shows suitable water solubility, givingoptimum imidization yield and high solid content of the finaldispersion. It has been surprisingly found that the anhydride monomercontent further determines the particle size, the particle sizeincreasing with increasing anhydride monomer content, as well as thehardness of the copolymer. The properties of the copolymer particlesafter imidization are determined not only by the composition of thestarting material, but also by the physical conditions prevailing in theimidization reaction, e.g. concentration and agitation.

The vinyl monomer content of the copolymer ranges between 95-50 mole %,preferably between 95-81 mole %.

The aqueous polymer dispersion of this invention preferably has a solidcontent of more than 20 wt. %, more than 30 wt. % or even more than 40wt. %. The dispersion comprises discrete particles having a particlediameter above 30 nm, sometimes above 40 or 50 nm, but smaller than 400nm, often smaller than 250 or even 120 nm, the particle sizedistribution being narrow. As the diameter of the particles is smallerthan the wavelength of visible light, a smooth, high gloss andtransparent coating may be obtained. By controlling the particle size,preference may be given to a coating with a higher or lower gloss, beingmore transparent or showing some opaqueness. The formation of smallparticles further entails the advantage that stabilisation of thedispersion can be dispensed with. This is in contrast to a dispersioncontaining larger particles which needs the presence of an emulsifier toattain a stable dispersion.

The present invention also relates to a process for the production ofthe above described aqueous polymer dispersion. According to thisprocess an aqueous polymer dispersion is prepared by

-   -   1) reacting a starting copolymer of anhydride monomer units        (preferably maleic anhydride) and vinyl monomer units        (preferably styrene) with an aqueous solution of NH₃ or an amine        (RNH₂),    -   2) subjecting the thus obtained reaction mixture to an        imidization reaction until at least 90 mole % of the anhydride        monomer units have been imidized.

To obtain a dispersion the particles of which have a Tg that is as highas possible, the imidization reaction is preferably continued until atleast 95 mole %, or even virtually all anhydride monomer units have beenimidized.

A copolymer containing vinyl monomer units and anhydride monomer unitsmay be synthesised according to processes well known to the man skilledin the art, such as for example the process described in Hanson andZimmerman, Ind. Eng. Chem. Vol. 49, nr. 11 (1957), p. 1803-1807.

In the method of this invention, the copolymer is reacted in water, anemulsifier optionally being present. To this mixture an aqueous solutionof NH₃ or an amine RNH₂ is added, in which R may be an alkyl grouphaving between 1-18 carbon atoms or an aryl group. It is howeverpreferred to use NH₃, although butylamine and stearylamine also appearto be suitable imidization reactants.

It is preferred to keep the excess of NH₃ or RNH₂ in the course of theimidizaton reaction as low as possible. To minimise the unnecessary lossof chemicals it is preferred that the molar ratio between the amine orNH₃ and the anhydride monomer in the copolymer to be imidized rangesbetween 0.8:1 and 1.2:1, but is preferably an equimolar ratio orslightly less. In the latter case, complete conversion of the reactiongives an odourless dispersion as all amine or NH₃ is consumed.

However, it is technically possible to choose the amount of NH₃ or RNH₂such that the upper limit of the molar ratio of (NH₃ or RNH₂):(anhydride monomer present in the copolymer to be subjected toimidization) is 10:1. The lower limit may be 0.5:1. Of course it istechnically feasible to keep the ratio close to equimolar ratio.

If so desired, the imidization reaction may be carried out in thepresence of an alkali salt of an acid functional polymer containing acidfunctional monomer units and vinyl aromatic monomer units. For examplealkali salt of styrene maleic anhydride copolymer may be used, whichpreferably has molecular mass of from 500 to 10000 g/mol and with maleicanhydride content of at least 30 mole %. The alkali salt may function asan emulsifying agent.

The anhydride monomer/vinyl monomer copolymer has a molecular weightwhich preferably is not too high and neither too low so as to allowobtaining a dispersion with a sufficiently high solid content. In thepresent invention, the anhydride monomer/vinyl monomer copolymer has amolecular weight which is at least 1000 g/mole, preferably at least10000 g/mole, more preferably at least 60000 g/mole. The molecularweight of this copolymer is preferably less than 500000, more preferablyless than 200000 g/mole or less than 150000 g/mole. Ideally, themolecular weight of the starting copolymer is between approximately50000 and 80000 g/mole as it allows obtaining so-called monodispersedispersion with a narrow particle size distribution of between 50 and100 nm, the mean particle diameter being approximately 70 nm. Ultimatelysuch dispersion allows obtaining a coating with an optimum gloss.

If so desired, the anhydride monomer/vinyl aromatic monomer copolymerused may be a copolymer composition comprising a plurality of copolymershaving varying molecular weights. This function may be fulfilled by theemulsifier. The molecular weight of the copolymer after imidization hasbeen found to be a key parameter when processing it.

A too high molecular weight of the copolymer involves the risk that theviscosity of the dispersion becomes too high and the solid content toolow. A too low molecular weight of the copolymer involves the risk thatthe solid content of the dispersion gets too high, which has an adverseeffect on the applicability of the dispersion. A too low molecularweight of the copolymer involves the additional risk to intra-particleadhesion and agglomeration, due to Van der Waals attraction between theparticles, involving entanglement and the formation of particles withtoo large dimensions.

In the method of this invention, the imidization reaction will mostly becarried out at a temperature above 100° C., preferably between 120-195°C., more preferably at a temperature between 130-180° C., or even150-175° C. Below 100° C. insufficient imidization has been observed. Ata temperature above 170° C. and in particular above 195° C., there is anincreasing risk to agglomeration of the polymer, as a consequence ofwhich particle formation in the dispersion is counteracted, givingparticles with a too large size which are visible when applied as acoating and easily involve film formation. Within the claimedtemperature ranges, the imidization reaction is favoured over theformation of an imine-amine compound. The temperature range of 130-180°C. is preferred as within this range a well-defined dispersion withrespect to Tg and mechanical properties and composition is obtained andthe process showing good reproducibility.

Also, within the claimed temperature ranges sufficient imidization canbe obtained within an economically feasible reaction time, at a pressurewhich is not too high, e.g. approximately 7 bar. The risk to formationof imine-amine compounds should be minimised as these compounds have alower glass transition temperature (Tg), thus giving rise to particlesthat are liable to film-formation in the course of a calendaringprocess.

To minimise adhesion of the reaction mixture to the reactor wall in thecourse of the imidization reaction, the reaction mixture is stirred. Ithas namely been found that after the aqueous solution of the copolymerhas been contacted with ammonia or the amine, in the course of theimidization reaction a gel phase is formed, which may be broken or cutthrough stirring, adhesion to the reactor wall thereby being minimised.This cutting action assists in shaping the particles of the dispersionformed following imidization.

The rotation speed applied upon stirring of the reaction mixture and thetime during which the reaction mixture Is stirred, will be in generaladapted by the man skilled in the art. Adapting this parameter allowcontrolling the physical properties and particle size of the dispersionobtained, i.e. allows controlling whether small particles with a smoothsurface are formed which ultimately give a coating with a high gloss andgood transparency, or larger particles if a more opaque coating is aimedat. Stirring assists in avoiding the formation of particles with a roughsurface and non-uniform shape which, when applied as a coating wouldgive undesired scattering. It has been observed that the more uniformthe shape of the particles, the better the gloss of the coating and thebetter the drainage properties of the coating when imprinted. The shapeof the particles is determined by forces prevailing in the course of theimidization reaction, and e.g. by the time the reaction mixture isstirred.

If so required, the imidization reaction may be carried out in thepresence of an anti-foaming agent and/or an emulsifier. Suitableemulsifiers may be anionic or nonionic surfactants.

With the above described production process, an aqueous dispersion ofthe imidized organic pigment may be obtained with the above-describedsolid content and particle size.

If desired, the solid content of the dispersion may be increased bymethods known by man skilled in the art, especially suitable areevaporation and ultra filtration.

When applied to a surface on top of one or more already existingcoatings, due to their small size, the particles of the dispersion ofthis invention are capable of filling gaps left in the already appliedcoating. In that way a surface may be obtained which is covered by anoptimum coating, providing optimum drainage properties that are hardlydisturbed by the underlying coatings.

As the size of the individual particles is relatively small, a densepacking of the particles is obtained when the dispersion Is applied to asurface to be coated, and dried. The small size of the particlesfacilitates solvent release and drying of the coating, thus minimisingthe risk to crack formation upon drying of the coating and improvingdrying time of the coating. The formation of small pigment particlesfurther has the advantage that inter-particle attraction is governed byVan der Waals forces, giving strong inter-particle adhesion and goodadhesion to the surface to be coated. Due to the dense packing, a closedtop coating is achieved which when imprinted shows good drainageproperties thus providing quick drying of the ink, even after forexample a coated paper has been subjected to a calendaring process. Thisis attributed to the formation of drainage channels upon drying of thecoating. The dense coating assists in minimising penetration of inkparticles to underlying coatings, as a consequence of which thesharpness and fineness with which the coated material is imprinted, thusthe over-all printing quality, is improved as compared to prior artcoatings. Furthermore, tearing of paper coated with the coatingcomposition of this invention and the occurrence of wet pick isdecreased.

It has been further found that with the method of this invention, theimidized pigment particles obtained are microporous. As the dimensionsof the micropores are small, penetration of ink particles into thepigment particles is inhibited, as a consequence of which the printingquality of a surface coated with a coating containing the dispersion ofthis invention, is further improved.

The present invention further relates to a coating composition for asurface to be coated, the coating composition comprising an amount ofthe aqueous dispersion of this invention. The amount of organic pigmentincorporated may vary within wide ranges and will mostly be determinedby the application. In case of cheap paper applications, low amounts oforganic pigments will be used so as to obtain a low density coating.When using a dispersion with a high concentration of organic pigment,either a more dense coating may be obtained, or the amount of dispersionused may be decreased.

The coating composition may further comprise the usual ingredients, suchas binders (starch, latex, polyvinylalcohol, etc) and conventionalpigments (kaolin, PCC, GCC, talc, silica, etc), which may be partlysubstituted by the polymer dispersion of this invention if so desired.The coating composition may further contain thickening agents.Hyperbranched polyesteramides such as those disclosed in U.S. Pat. No.6,392,006 may be added to control the viscosity of the coatingcomposition.

It has been found that the aqueous polymer dispersion of the presentinvention is a suitable coating material for a wide variety of surfacesthat are to be imprinted. For example, the aqueous dispersion of thisinvention appears suitable for coating paper, paperboard, cardboard, anorganic film (for example a polyethylene film), a metal foil, a textilesheet, etc. When coated with a coating comprising the aqueous polymerdispersion of this invention, an increased gloss of coated paper of 5-10points after calendaring has been observed. This gloss improvement maybe further increased by using a coating which exclusively consists ofthe dispersion of this invention.

The extent to which the anhydride monomer/vinyl monomer copolymer hasbeen imidized determines the acidity of the imidized copolymer.Controlling the pH allows controlling foam formation in the course ofthe coating process. This is an advantage as compared to known coatingcompositions as they often contain calcium carbonate showing superfluousfoaming. The pH further determines the area in which the dispersion ofthis invention may be applied. The dispersion of this invention has beenfound to have a pH close to 7.

EXAMPLES

Characterisation Methods:

PCS Measurements

The average hydrodynamic radius of the particles of the dispersion afterimidization was determined using Photon Correlation Spectroscopy.

Measurements were carried out using an ALV Laser of theVertriebsgesellschaft mbH, Langen, Germany.

Solid Content.

The solid content was determined using an infrared instrument, typeMettler LP16/PM600.

pH Measurements.

The pH value of each sample was measured with a Knick 752 Cl, nr. 051489pH measurement instrument.

Determining the Degree of Imidization.

The degree of imidization may for example be determined with Raman FTIRspectroscopy, by correlating the absorption intensity to the intensityof the absorption at the same wavelength of a completely imidized and anon-imidized reference sample. Before carrying out any calculations, theRaman-FTIR signals were normalised based on the absorption signalsoriginating from the aromatic rings in the polymer chains. Thecalculations were based on the following absorptions:

-   C═O imide absorption band, relatively intense signal at    approximately 1768 cm⁻¹-   C═O anhydride absorption band, at approximately 1860 cm⁻¹-   C═O relatively weak absorption band of carboxylic acid groups, at    approximately 1715 cm⁻¹

As a reference use was made of (1) an aqueous ammonia solution of animide free polymer, prepared starting from 26 mole % of maleic anhydride(MA) and 74 mole % of styrene, a NH₃:maleic anhydride ratio of 3:1, at50° C.; (2) a SMA powder that had been subjected to an imidizationreaction by mixing 2 g of SMA (28 wt. % of MA, 72 wt. % of styrene;molecular weight 110000 g/mole) with 0.50 g of ureum in a double vicemini extruder, at 240° C. for 5 minutes at a rotation speed of 100 rpm.

Contact Angle Measurements

Contact angles were measured with a contact angle meter type Digidrop,GBX, Roman, France.

Example 1

140 g ground SMA and water were charged into a double walled, oil heatedreactor of 1 l, which contained a stirrer. The SMA had a MA content of26 mole % and a molecular weight of 80000 g/mole. To this solution a 25%NH₃ solution was added, so that the MA:NH₃ ratio was 1:1. Furthermore, apotassium salt of a SMA polymer was added having a molecular weight of1000 g/mole and a MA content of 48 mole %. The K salt:SMA ratio was0.03:1. Water was added until a total volume of 700 ml was obtained. Thepressure was adjusted to 0.2 MPa with nitrogen. Following increasing thetemperature to 160° C., at a rotation speed of 800 rpm, the pressureraised to 0.8 MPa. After 6 hours of reaction time a polymer dispersionwas obtained having a solid content of approximately 20 wt. %, theparticle size being between 80 and 120 nm. The MA had been completelyconverted to imide. The Tg of the polymer after completion of theimidization was found to be between 190 and 200° C. The dispersion had apH of 6.8. The contact angle of the dispersion when applied to paper wasfound to be smaller than 40°.

Example 2

140 g ground SMA and water were charged into a double walled, oil heatedreactor of 1 l, which contained a stirrer. The SMA had a MA content of26 mole % and a molecular weight of 80000 g/mole. To this solution a 25%NH₃ solution was added, so that the MA:NH₃ ratio was 1:1. Water wasadded until a total volume of 700 ml was obtained. The pressure wasadjusted to 0.2 MPa with nitrogen. Following increasing the temperatureto 160° C., at a rotation speed of 800 rpm, the pressure raised to 0.8MPa. After 6 hours of reaction time a polymer dispersion was obtainedhaving a solid content of approximately 20 wt. %, the particle sizebeing between 80 and 120 nm. The MA had been completely converted toimide. The Tg of the polymer after completion of the imidization wasfound to be between 190 and 200° C. The dispersion had a pH of 7.0. Thecontact angle of the dispersion when applied to paper was found to besmaller than 40°.

Example 3

245 g ground SMA and water were charged into a double walled, oil heatedreactor of 1 l, which contained a stirrer, to which 0.2 g of Surfinol420 of Air Products was added. The SMA had a MA content of 26 mole % anda molecular weight of 80000 g/mole. To this solution a 25% NH₃ solutionwas added, so that the MA:NH₃ ratio was 1:1. Furthermore, a potassiumsalt of a SMA polymer was added having a molecular weight of 1000 g/moleand a MA content of 48 mole %. The K salt:SMA ratio was 0.03:1. Waterwas added until a total volume of 700 ml was obtained. Followingincreasing the temperature to 160° C., at a rotation speed of 800 rpm,the pressure raised to 0.6 MPa. After 6 hours of reaction time a polymerdispersion was obtained having a solid content of approximately 35 wt.%, the particle size being between 80 and 120 nm. The MA had beencompletely converted to imide. The Tg of the polymer after completion ofthe imidization was found to be between 190 and 200° C. The dispersionhad a pH of 6.9. Upon drying of the coating no film formation wasobserved. The contact angle of the dispersion when applied to paper wasfound to be smaller than 40°.

Example 4

An experiment was carried out on pilot scale, by dissolving 60 kg ofground SMA type Stapron 28110 (DSM, The Netherlands) in 99 kg of waterin a pilot reactor at room temperature, to which 0.02 kg of Surfinol 420of Air Products was added. The SMA had a MA content of 28 mole % and amolecular weight of 110000 g/mole. To this solution 11.70 kg of a 25%NH₃ solution in water was added, so that the MA:NH₃ ratio was 1:1. Whenadmitting the NH₃ solution, the temperature was increased toapproximately 78° C. The reactor was further heated until the reactionmixture had a temperature of approximately 155° C. In the course of thereaction, the reaction mixture was stirred using a motor of 35 kW with areduction of 56 rpm. The electric power needed to drive the stirrer wasrecorded as a function of time. The results are summarised in Table 1.It was observed that as soon as the reaction mixture obtained atemperature of approximately 134° C., the viscosity increasedsignificantly and a first gel was formed, indicating that theimidization reaction was started by formation of the amide compounds.When continuing the reaction, the reaction mixture became visco-elastc,indicating the formation of the imide was taking place. Formation of SMIparticles was observed after a reaction time of approximately 210minutes, at the moment a significant decreasing viscosity was observed.It was further observed that as soon the pH of the reaction mixtureobtained a value of approximately 7, the imidization was complete. Aftera reaction time of approximately 4 h and 15 min, heating was stopped andthe reaction mixture was cooled down to room temperature. A dispersionof SMI in water was obtained having a solids content of 40 wt. %, a pHof 7, a mean particle diameter of 86 nm. TABLE 1 Reaction Reaction timetemperature Electric power Pressure (min) (° C.) (Ampère) Rpm (bar)Observations 0 23 2 25 0.50 Introduction of water and Stapron 15 78 4 350.50 Introduction of NH₃ solution - closing reactor 30 134 25 37 0.70Increasing viscosity, vortex decrease 45 140 30 46 1.00 Idem 60 143 4050 1.10 No vortex - high viscosity 75 150 50 50 1.15 Idem 90 154 50 501.15 Increase visco elasticity 105 158 55 50 1.80 Idem 120 157 58 502.00 High gloss - jelly aspect 135 156 63 50 2.00 Maximum shearresistance 150 155 63 50 2.70 Maximum shear resistance 165 155 54 503.00 Decreasing viscosity 180 155 48 50 3.30 Idem 195 155 38 50 3.40Vortex coming back 210 155 29 25 3.40 Pressure increased 1 bar manually225 155 18 20 3.50 Stop heating - start cooling 240 100 2 20 3.70 Lessfoam - viscosity like water 255 45 2 15 3.80 Dispersion - end of process

Example 5

The procedure of Example 4 was repeated, this time dissolving 1400 kg ofground SMA type Stapron 26080 (DSM, The Netherlands) in 2352 kg of waterin a pilot reactor at room temperature, to which 0.05 kg of Surfinol 420of Air Products was added. The SMA had a MA content of 26 mole % and amolecular weight of 80000 g/mole. To this solution 249 kg of a 25% NH₃solution in water was added, so that the MA:NH₃ ratio was 1:1. Whenadmitting the NH₃ solution, the temperature was increased toapproximately 86° C. The reactor was further heated until the reactionmixture had a temperature of approximately 150° C. The electric powerneeded to drive the stirrer stirring the reaction mixture was recordedas a function of time. The results are summarised in Table 2. It wasobserved that as soon as the reaction mixture obtained a temperature ofapproximately 101° C., the viscosity increased significantly and a firstgel was formed, indicating that the imidization reaction was started byformation of the amide compounds. When continuing the reaction, thereaction mixture became visco-elastic, indicating the formation of theimide was taking place. Formation of SMI particles was observed after areaction time of approximately 5 hours, at that moment a decreasingviscosity was observed. It was further observed that as soon the pH ofthe reaction mixture obtained a value of approximately 7, theimidization was complete. After a reaction time of approximately 6 h and45 min, heating was stopped and the reaction mixture was cooled down toroom temperature. A dispersion of SMI in water was obtained having asolids content of 40 wt. %, a pH of 7, a mean particle diameter of 72nm. TABLE 2 Reaction Reaction Electric Time temperature power Pressure(min) (° C.) (Ampère) Rpm (bar) Observations 0 23 2 50 0.50 Introductionof H₂O 15 75 4 35 0.50 Introduction of Stapron 26080* 60 86 7 37 0.70Introduction of 25% NH₃ 75 101 10 46 1.00 Start of increase of viscosity90 108 15 50 1.10 Strong viscosity increase 105 114 15 50 1.15 Idem 120116 15 50 1.15 Idem 135 118 17 50 1.80 Idem 150 122 17 50 2.00 Idem 165124 17 50 2.00 Increase of visco elasticity 180 127 22 50 2.70 Decreaseof vortex 195 131 22 50 3.00 High gloss and jelly aspect 210 135 22 503.30 Slow movement of gel phase in the reactor 225 137 24 50 3.40 Novortex 240 138 24 50 3.40 No vortex 255 140 24 50 3.50 No vortex 270 14135 50 3.70 No vortex 285 142 35 50 3.80 Forming of chewing gum bubbles300 144 35 50 3.80 Idem 315 145 38 50 3.80 Idem 330 146 39 50 3.80 Idem345 147 40 50 3.80 Idem 360 149 42 50 4.00 Very high shear resistance375 150 43 50 5.00 Maximum sheer resistance 390 150 43 50 5.00 Maximumsheer resistance 405 149 43 50 5.00 Maximum sheer resistance 420 149 4050 5.00 Viscosity decrease 435 150 39 50 5.00 Viscosity decrease 450 15036 50 5.00 Light foam and vortex forming 465 153 32 25 5.00 Strong foamforming 480 154 22 25 6.00 Manuel addition of +1 bar pressure 495 154 1225 6.00 Decrease of foam forming till 6 bar 510 155 8 25 6.00 Lowviscosity and minimum foam 525 155 7 25 6.00 Stop the heating and startcooling 540 100 5 25 6.00 Less foam and viscosity like water 555 40 2 256.00 Dispersion - end of process

Example 6

The experiment of Example 1 was repeated using SMA type 2000 (fromAtofina, France). The resulting dispersion had a solid content ofapproximately 20 wt % and a pH of 7.1. The particle size was 1500 nm.

Example 7

Approximately 3 kg of the product obtained from Example 5 was subjectedto ultrafiltration using a Valmet Flootek CR 200/1 with a 30000 Daltoncut-off. The final stable dispersion had a 58.6% solid content.

Example 8

Wood free paper with dry weight of 67 g/m² was coated with an on-linefilm press at a speed of 1300 m/min with base coating of 5 g/m² on bothpaper sides calculated as dry matter. The applied coating had a solidcontent of 60 wt. % and contained 100 parts calcium carbonate, 8 partslatex and 8 parts starch. On top of the base coating, an mid coating of13 g/m² calculated as dry matter was applied off-line on both papersides having the same composition as the base coating, however with asolids content of 64 wt. %.

The thus pre-treated paper was coated on both sides with a top coatingof 6 g/m² calculated as dry matter. The top coating had the followingcomposition: 75 parts of CaCO₃, 25 parts of kaolin, 14 parts of latex,the usual additives and 10 parts of the polymer dispersion of thisinvention, the properties of which are given in Table 3 below.

The paper was passed through a calendar having 9 successive nippassages, at a nip pressure of 220 N/m.

The thus obtained paper had a total dry weight of 115 g/m².

Comparative Example

The experiment of Example 8 was repeated, however this time using as atop coat a reference coating containing 75 parts of CaCO₃, 25 parts ofkaolin, 14 parts of latex and the usual additives. The reference coatingdid not contain the polymer dispersion of this invention. Thereafter thepaper was passed through a calendar having 11 successive nip passages,at a pressure of 280N/m at each nip.

The thus obtained paper had also a total dry weight of 115 g/m² as thedry weight of the base paper and the amount of coatings was the same asin Example 8. TABLE 3 Particle size Dispersion ¹⁾ (nm) Paper gloss 2608080 77 28110 95 75 33003 1500 57 34080 150 73 Comp. Ex. 75¹⁾SMA notation, first two numbers denote the MA content, last threenumbers Mw × 10⁻³.

From the comparison of the results of Example 8 with the Comparativeexample it becomes clear that with the polymer dispersion of thisinvention a similar gloss can be obtained although both the number ofcalander nips and the nip pressure used is lower.

It was further observed that the paper obtained according to thisinvention was thicker and had higher Scott Bond values as compared tothe paper obtained with the Comparative example. This is attributed tothe fact that with the invention the number of nips required in thecalendaring process may be reduced as well as the nip pressure, as aconsequence of which the paper strength is less affected duringcalendaring and a high initial gloss may be obtained.

1. An aqueous polymer dispersion comprising a copolymer of anhydridemonomer units and vinyl monomer units, which copolymer has beensubjected to an imidization reaction, characterized in that at least 90mole % of the anhydride monomer units of the copolymer are imidized. 2.An aqueous dispersion as claimed in claim 1, characterized in that theanhydride monomer content of the copolymer ranges between 5-50 mole %and the vinyl monomer content of the copolymer ranges between 95-50 mole%.
 3. An aqueous dispersion as claimed in claim 1, characterized in thatthe copolymer has a molecular weight ranging between 1000-500000 g/mole.4. An aqueous dispersion as claimed claim 1, characterized in that thecopolymer is a copolymer composition comprising a plurality ofcopolymers having varying molecular weights.
 5. An aqueous dispersion asclaimed in claim 1, characterized in that the dispersion has a solidcontent of more than 20 wt. %.
 6. An aqueous dispersion as claimed inclaim 1, characterized in that the polymer dispersion comprises discreteparticles having a particle size between approximately 30-400 nm.
 7. Anaqueous dispersion as claimed in claim 1, characterized in that thecopolymer contains maleic anhydride monomer units and styrene monomerunits.
 8. A method for the production of an aqueous polymer dispersioncomprising the steps of 1) reacting a starting copolymer of anhydridemonomer units and vinyl monomer units in an aqueous solution of NH₃ oran amine RNH₂, 2) subjecting the thus obtained mixture to an imidizationreaction, characterized in that the imidization reaction is carried outunder reaction conditions which are selected so that at least 90 mole %of the anhydride monomer units have been imidized.
 9. A method asclaimed in claim 8, characterized in that the imidization reaction iscontinued until a degree of imidization of the copolymer of at least 95mole % is obtained.
 10. A method as claimed in claim 8, characterized inthat the imidization reaction is carried out in the presence of analkali salt of an acid functional polymer containing acid functionalmonomer units and vinyl aromatic monomer units.
 11. A method as claimedin claim 8, characterized in that the copolymer has a molecular weightranging between 1000-500000 g/mole.
 12. A method as claimed in claim 8,characterized in that NH₃ or RNH₂ is added in such an amount that theratio of NH₃ or amine:anhydride monomer in the starting copolymer isbetween 0.5-10:1.
 13. A method as claimed in claim 8, characterized inthat the molar ratio between the amine or NH₃ and the anhydride monomerin the copolymer ranges between 1.2-0.8:1.
 14. A method as claimed inclaim 8, characterized in that in the course of the imidizationreaction, the reaction mixture is stirred so as to minimize adhesion ofthe reaction mixture to the reactor wall.
 15. A method as claimed inclaim 8, characterized in that the imidization reaction is carried outat a temperature above 100° C.
 16. An aqueous coating composition forcoating a product to be imprinted, characterized in that the coatingcomposition contains a polymer dispersion as claimed in claim
 1. 17. Anaqueous coating composition according to claim 16, further containingbinders, conventional pigments and, optionally, additives. 18-19.(canceled)
 20. A method of coating a surface to be imprinted comprisingapplying to said surface a polymer dispersion according to claim
 1. 21.A method according to claim 20, characterized in that the surface ispaper, paperboard, cardboard, organic film e.g. polyethylene film, metalor textile.